Motor Load Calculations, Part 3

In Part 2, we said you needed to use Tables 430.247, 430.248, 430.249, and 430.250 as appropriate. How do you know what’s appropriate? That is, which table is the right one?

If you look at the table titles, you’ll see each one describes a voltage class. So if, for example, your motor is supplied by 3-phase AC then you’ll use Table 430.250. This table applies to the typical industrial motor.

If you look at the voltages listed in each table, you might conclude that the NEC requires you to use one of those voltages in your load calculations. But that is not the case. Instead, refer to 220.5 and choose from the nominal voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts. Rather than correspond to the nominal voltage, these 430 tables correspond to a different quantity—in the case of phase AC motors, it’s the rated motor voltage.

But if you’re working in Article 430, why would it occur to you to look up some requirement back in Chapter 2? Because Article 220 provides the load calculation requirements for branch, feeder, and service loads. Remember, the first four Chapters of the NEC apply to all installations except as amended by later Articles for specific types of installations.

Always start with the universally applicable requirements and then look to the specific case requirements to see if they’ve been amended. Just keep in mind that some Chapter 4 Articles apply to specific types of installations.

« Part 2Part 4 » | Source: Mark Lamendola | Mindconnection

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